1. Field of the Invention
The present invention relates to an optical fiber and an optical transmission line for a long-haul optical transmission.
2. Description of the Related Art
In a long-haul optical transmission line, a technology for enhancing the characteristics of a transmission line is used in a positive manner by building an optical transmission line combining two types of optical fibers having different optical characteristics. A dispersion-managed optical fiber transmission line and a nonzero dispersion-shifted optical fiber transmission line are representative examples of the technology (see, for example, M. Suzuki, et al., “Dispersion-Managed High-Capacity Ultra-Long-Haul Transmission”, J. Lightwave technol., vol. 21, no. 4, pp. 916-929, April 2003). In the dispersion-managed optical fiber transmission line, the overall wavelength dispersion is managed to be zero by combining a positive dispersion fiber having a positive wavelength dispersion and a negative dispersion fiber having a negative wavelength dispersion. The nonzero dispersion-shifted optical fiber transmission line is configured by combining a large-effective area (Aeff)-type nonzero dispersion-shifted optical fiber having a large effective core area and a low-slope-type nonzero dispersion-shifted optical fiber having a small dispersion slope. The nonzero dispersion-shifted optical fiber is a single-mode optical fiber having a small wavelength dispersion at a wavelength of a signal light, for example, about −5 ps/nm/km to −2 ps/nm/km or about 2 ps/nm/km to 5 ps/nm/km, and a nonzero dispersion-shifted optical fiber having a negative dispersion is referred to as a negative-dispersion nonzero dispersion-shifted optical fiber. The negative-dispersion nonzero dispersion-shifted optical fiber is widely used for a submarine cable.
The optical characteristics of the large-Aeff-type nonzero dispersion-shifted optical fiber show, for example, an effective core area of 75 μm2, a dispersion slope of 0.10 ps/nm2/km, and the optical characteristics of the low-slope-type nonzero dispersion-shifted optical fiber show, for example, an effective core area of 50 μm2, a dispersion slope of 0.05 ps/nm2/km. After all, the average optical characteristics of a negative-dispersion nonzero dispersion-shifted optical fiber transmission line built by connecting virtually the same lengths of the above optical fibers show an effective core area of 65 μm2, a dispersion slope of 0.07 ps/nm2/km.
Most commonly, in the nonzero dispersion-shifted optical fiber transmission line, the optical signal is transmitted from the large-Aeff-type nonzero dispersion-shifted optical fiber side. As a result, in a state in which the optical intensity of the optical signal is large, the large effective core area of the optical transmission line effectively suppresses an occurrence of the nonlinear optical phenomena. After that, the optical intensity of the optical signal is attenuated due to a transmission loss of the optical fiber, and then the optical signal is input into the low-slope-type nonzero dispersion-shifted optical fiber. Although the low-slope-type nonzero dispersion-shifted optical fiber has a relatively small effective core area, the wavelength dispersion is less dependent on the wavelength owing to a small dispersion slope. As a result, in the case of transmitting a wavelength-division-multiplexed (WDM) optical signal in which optical signals having different wavelengths are wavelength multiplexed, an occurrence of a discrepancy of the wavelength dispersion between the optical signals can be effectively suppressed.
In other words, there is a tradeoff relationship between the effective core area and the dispersion slope in the nonzero dispersion-shifted optical fiber. For this reason, as described above, it is attempted to relieve the tradeoff relationship as the whole optical fiber transmission line in the nonzero dispersion-shifted optical fiber transmission line by arranging a nonzero dispersion-shifted optical fiber having a large effective core area at an earlier stage of transmitting an optical signal and arranging a nonzero dispersion-shifted optical fiber having a small dispersion slope at a later stage.
On the other hand, in the dispersion-managed optical transmission line, a technology of greatly enlarging the effective core area is disclosed, by employing a multimode optical fiber as the negative-dispersion fiber (see, for example, Japanese Patent Application Laid-open Publication No. 2004-271904).
However, particularly in a nonzero dispersion-shifted optical fiber transmission line used for a submarine application, an even longer transmission distance is required, but there is a problem that an accumulation of the discrepancy of the wavelength dispersion between the optical signals gets in the way of extending the transmission distance. On the other hand, if the optical fiber is designed to have a small dispersion slope to resolve the discrepancy of the wavelength dispersion between the optical signals, the effective core area is decreased, which causes a problem of a remarkable occurrence of the nonlinear optical phenomena.